OMAP3: PM: Remove un-necessary cp15 registers form low power cpu context

The current code saves few un-necessary registers which are read-only or
write-only, unused CP15 registers.

Remove them and keep only necessary CP15 registers part of
low power context save/restore.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>